2014A&A...561A..40M

The supernova SN 1993J in M81 is the most extensively studied young radio-luminous supernova in the northern hemisphere. We recently reported results from the analysis of a complete set of very-long-baseline-interferometry (VLBI) observations of this supernova at 1.7, 2.3, 5.0, and 8.4GHz, covering a time baseline of more than one decade. These results focused on the kinematics of the expanding shock, the particulars of its evolving non-thermal emission, the density profile of the circumstellar medium, and the evolving free-free opacity by the supernova ejecta. In the present paper, we complete our analysis by performing a search for any possible signal from a compact source (i.e., a stellar-mass black hole or a young pulsar nebula) at the center of the expanding shell. We have performed a stacking of all our VLBI images at each frequency, after subtraction of our best-fit shell model at each epoch, and measured the peak intensity in the stacked residual image. Given the large amount of available global VLBI observations, the stacking of all the residual images allows us to put upper limits to the eventual emission of a putative compact central source at the level of ∼102 µJy at 5GHz (or, more conservatively, ∼192 µJy, if we make a further correction for the ejecta opacity) and somewhat larger at other wavelengths.